It is well known that cardiac arrhythmias such as ventricular fibrillation may be controlled with devices such as implantable defibrillators. Many different types of defibrillation electrodes have been suggested over the years, as can be seen from the following examples. In this discussion, no distinction will be made between cardioversion and defibrillation; both will be referred to as defibrillation.
U.S. Pat. No. 3,942,536 issued to Mirowski et al. discloses an intravascular bipolar catheter electrode system wherein each of two electrodes is composed of a plurality of spaced, low impedance rings. As implanted, the first electrode is located within the right ventricle (RV) and the second electrode is located in the superior vena cava (SVC).
In U.S. Pat. No. 4,161,952 issued to Kinney et al., a catheter electrode has a coil of wound spring wire, with filler material beneath and between individual turns of coil such that only the outside of the wound wire is exposed to the patient's body. It is designed to reside in or about the heart, as in the SVC or in the coronary sinus (CS).
U.S. Pat. No. 4,922,927 issued to Fine et al. teaches the use of tightly wound wire forming a tight coil on a support that is flared to provide a greater diameter along its midsection than at its ends, to form an RV electrode. A copper-zirconium alloy wrapped with tantalum and coated with iridium oxide is suggested for the tightly wound wire.
Other types of transvenously placed leads are disclosed in U.S. Pat. No. 4,998,975 issued to Cohen et al. One lead is placed through the heart wall, and into the pericardial space, and another is placed endocardially in a conventional manner. Both leads are shown with several embodiments, with the examples of general electrode construction being to expose a section of the conductor coil, or to use ring electrodes similar to those used in conventional bipolar pacemaker leads. Cohen et al. also describe two methods for steering more current to a selected region of the heart. The first method is to apply various voltages to the connectors of each of four electrodes. The second method uses the resistance of conductors, both between connector and electrode, and between two electrodes on the same lead, and the body tissue resistance between electrodes on different leads, to form a voltage divider, thus creating a different potential at each electrode.
Another lead system patent, U.S. Pat. No. 5,007,436 issued to Smits, describes electrodes of both J and straight configurations, for use in the RV, right atrium, great cardiac vein, or CS. The fabrication methods suggested use close wound conductive coils mounted exterior to an elongated insulative sheath, or the method of Kinney et al.
Spiral shaped electrodes for endocardial, epicardial, or extrapericardial implantation are described in Heil, Jr. et al., U.S. Pat. No. 5,016,808, Fogarty et al., U.S. Pat. No. 4,860,769, and Hauser et al., U.S. Pat. No. 5,052,407. The electrodes of these patents use various construction techniques, including electrodeposition or vapor deposition onto a plastic tube, helically wound wire (round or ribbon, unifilar or multifilar, single or double helix) or conductive rings on a flexible insulating portion, and conductive screen wrapped around a tubular body.
Other defibrillation leads are disclosed in Mehra et al., U.S. Pat. No. 5,144,960, and in Bardy et al., U.S. Pat. No. 5,174,288.
Endotak SQ Model 0048 (Cardiac Pacemakers Inc., St. Paul, Minn., USA), described in "A Subcutaneous Lead Array for Implantable Cardioverter Defibrillators" by Jordaens et al., published in PACE, Vol. 16, July 1993, Part I, is an electrode system consisting of three conductive elements that can be subcutaneously inserted. The conductive elements of this "array lead" are made of electrically common multifilar coil, joined in a silicone yoke, and separately introduced with a lead tunneler and peel-away sheaths.
As defibrillator technology improves and the demand for defibrillators increases, it becomes increasingly desirable to have leads available that are easily implanted. Any implantable defibrillation electrode must be capable of withstanding repeated flexing over a long period of time. In addition, the electrode must have sufficient surface area to discharge high amounts of energy for effective defibrillation, and must maintain its electrical integrity. The electrode must be of biocompatible materials, as well as of a shape that avoids tissue damage.